Description of the Related Art
The application of radiation is used for a variety of diagnostic and therapeutic purposes. For example, external radiotherapy known as “teletherapy” is used to treat approximately half of all patients with cancer in the United States, as well as being used to treat patients with arterio-venous malformations, intraocular subfoveal neovascular membranes and Parkinson's disease, among other diseases and conditions.
Generally, teletherapy has been performed using x-ray beams or electron beams. More recently, however, teletherapy has been performed using proton beams due to two characteristics of proton beams. First, proton beams do not scatter as much as either x-ray beams or electron beams. Thus, teletherapy with a proton beam can be applied with a steeper dose gradient near the edge of the proton beam than for an x-ray beam or electron beam. Second, protons lose energy at a more rapid rate as they penetrate tissue, thereby delivering a greater dose at the depth of the target tissue. These two characteristics of proton beams allow the delivery of higher doses to target tissues while minimizing radiation to adjacent normal tissues.
The delineation of target tissues from non-target tissues and the selection of beam directions is typically performed using a computerized treatment planning system. The computerized treatment planning system analyzes input information, such as x-ray axial computed tomography and magnetic resonance imaging, and provides output information, such as beam directions, shapes of normal tissue shields for each beam, and patient alignment information for each beam.
Regardless of the type of teletherapy, however, proper patient alignment is critical to delivering sufficient radiation to target tissues while minimizing radiation delivered to non-target tissues. Patient alignment is the process by which a patient is reproducibly interfaced with the radiation delivery equipment for the purposes of obtaining anatomical, morphological, and physiological information, for performing treatment simulations, and for delivering treatments. The goals of patient alignment are to permit unrestricted access to the patient by radiation beams, and to provide accurate tissue targeting and dose delivery, while promoting patient comfort and safety, and allowing for quick patient egress from the radiation delivery equipment.
The five steps in the patient alignment process are registration, immobilization, localization, positioning and verification. Registration comprises placing the patient on a patient positioner, such as a movable table, in a reproducible manner. Immobilization comprises fixing the registered patient to the patient positioner so that they move together as a single unit in a controlled fashion. Localization comprises determining the location of the target tissue relative to the diagnostic, simulation or treatment unit. Positioning comprises moving the patient positioner to place the target tissue in the desired orientation at the desired location. Verification comprises verifying the patient's orientation and location, and can comprise using the same technique as localization. One or more than one of these steps can be repeated as required. If patient alignment is performed rapidly, the patient is more likely to remain properly aligned, minimizing the margin placed around the target tissue to account for motion and reducing the radiation dose to non-target tissues
Patient alignment is usually performed with the patient in a supine position because a larger surface area of the patient is captured by registration and immobilization devices, because the entire patient is at a height more accessible to treatment personnel and because patients are generally more comfortable in the supine position. Most patient positioners have, therefore, been some form of a table.
Registration is typically accomplished using a registration device such as a low-density foam that is custom molded to the patient's shape and attached to the top of the patient positioner. The patient lies directly on the foam, preventing the patient from rolling and translating with respect to the patient positioner, and increasing patient comfort.
Immobilization is typically accomplished using a thermoplastic net that attaches to the patient positioner and that covers both the patient and the registration device. Alternatively, for teletherapy involving the head and neck, immobilization can be accomplished using a ring referred to as a ‘halo’ that is screwed into the patient's skull and then bolted to the patient positioner.
High precision localization and verification generally rely on radiographic techniques and fiducial markers. The fiducial markers can be internal, such as natural anatomical landmarks or implanted landmarks, or can be external such as a z-box attached to a halo.
Localization and verification for proton beam teletherapy typically uses proton beam treatment units that comprise a diagnostic x-ray source capable of projecting an x-ray beam to simulate the intended path of the proton beam. The x-ray beam passes through the patient creating localization images captured on film or by an electronic portal imaging device. Localization is achieved by comparing the localization images with digitally reconstructed radiographs (DRRS) generated by the treatment planning system. The patient is repositioned iteratively and new localization images are generated until coincidence of the localization images and digitally reconstructed radiographs are obtained thereby verifying the location.
After patient alignment has been completed, teletherapy is commonly performed using isocentric gantries that facilitate the entry of radiation beams into patients from multiple directions in a timely manner. A gantry is a mechanical device that houses a radiation beam delivery system, and comprises one or more than one instrument, such as a particle accelerator, an x-ray tube, a beam spreading device, beam limiting collimators, a particle range modifier, a fluence modifying device and a dose monitoring detector.
The rotation axes of the gantry and the patient positioner intersect at a point in space called the isocenter. The center of the target tissue within the patient is generally placed at the isocenter. Unfortunately, radiation beam delivery devices within the gantry are prone to flex when rotated and, thereby, cause misalignment of the radiation beam with the target tissue.
Historically, when radiation field alignment was not critical to avoid normal tissues adjacent to the target tissues, the edges of radiation fields were placed at large distances around the target tissue volumes to ensure that the target tissue would be hit regardless of the misalignment of the radiation beam due to deflections of the radiation beam delivery system. When critical normal tissues were adjacent to target tissues, however, precise alignment was achieved either by radiographically repositioning the patient for each individual beam or by using large, rigid, and complex mechanical structures to reduce deflections of radiation beam delivery system. Disadvantageously, however, radiographically repositioning a patient requires at least about 15 minutes to align each radiation beam prior to radiation delivery. Therefore, delivering six beams to a patient requires a total treatment time of at least about 1.5 hours. Hence, radiographically repositioning a patient for each radiation beam significantly limits the number of patients that can be treated by each treatment apparatus and increases the cost per treatment.
Therefore, it would be useful to have a method of aligning a patient for delivering multiple radiation beams, such as proton beams, that allows a patient to be aligned in less time between beam deliveries. Further, It would be useful to have a device for aligning a patient for delivering multiple radiation beams, such as proton beams, that allows a patient to be aligned in less time.